The carbon nanotube is a carbon material with the feature of typical layered hollow structure, and the body of carbon nanotube is composed of a structural unit of hexagonal graphite carbon ring, which is a one-dimensional quantum material with special structure (nanometer scale in radial dimension, and micron dimension in axial dimension). Its tube wall is mainly composed of several layers to tens of layers of coaxial circular tubes, and there is a fixed distance of about 0.34 nm between layers; generally, its diameter is 2˜20 nm. Its conjugative effect is obvious since P electron on the carbon atom on the carbon nanotube forms a wide range of delocalized π bond; and it has excellent electrical properties for the structure of carbon nanotube is the same as the lamellar structure of graphite. The single-wall carbon nanotube material is regarded as a transparent electrode that can replace ITO especially by the research community and the industrial circle due to its high electronic mobility and low electrical resistivity. However, the strong van der waals force (˜500 eV/μm) and great slenderness ratio (>1000) between single-wall carbon nanotubes is usually easy to form a big tube bundle, which is hard to disperse, so its excellent performance and practical use are restricted greatly.
For carbon nanotubes grown by CVD, a staggered network structure is formed between tube bundles. Generally, many amorphous carbons and graphite debris may cover on the surface of tube bundles of the single-wall carbon nanotube, and the end of carbon nanotube is connected with catalyst, etc. Impurities in the network will influence the performance of carbon nanotube. So, the purification and dispersion methods of carbon nanotube play a very critical role in its application and development.
Main techniques of purifying and dispersing the carbon nanotube through wet chemical method include non-covalent functionalization, covalent functionalization and solvent stripping. The common character of these main techniques is that they all need the aid of a greater mechanical force. For example: use high frequency ultrasound, ball milling, etc. to promote the dispersion of carbon nanotube, then remove big tube bundles by high speed centrifugal separation. But large mechanical forces will damage carbon nanotubes inevitably, so separation of big tube bundles by high speed centrifugation will cause loss of carbon nanotubes (dispersion of carbon nanotubes only depending on mechanical forces may have a poor effect: most of carbon nanotubes aggregate together, and in the process of centrifugation, they may aggregate again; while if using high speed separation to remove big tube bundles, most of carbon nanotubes will be centrifuged). Additionally, while using the above described method, it is necessary to add surfactants such as sodium dodecyl benzene sulfonate, octyl phenol macrogol ester, polyvinyl pyrrolidone in water or alcohol or other common solvents to help disperse the single-wall carbon nanotube. The amount of the used surfactants is large, some reach above 10% in concentration, but the concentration of single-wall carbon nanotubes that can be dispersed is still low. The membrane conductivity and the heat transfer property of single-wall carbon nanotube are poor due to the influence of large amount of surfactants in the dispersion liquid. The use of non-covalent functionalization may introduce additives that help disperse carbon nanotubes such as surfactants or polymers etc. which are difficult to remove completely, thus reducing the electrical and thermodynamic properties of the carbon nanotube network itself; the use of covalent functionalization will damage the sp2 structure of functional site of carbon nanotube; and for solvent stripping, solvents reported currently have a high boiling point, high toxicity and low dispersion efficiency. Therefore, while maintaining the structural integrity of carbon nanotube and without adding any additive, effective dispersion of carbon nanotubes in common solvents is still an important issue in the research and application of single-wall carbon nanotube.
The conductivity of single-wall carbon nanotube is the highest among carbon nanotubes. But, the single-wall carbon nanotube is the most difficult one to be dispersed in various solvents. At present, water or alcohol with low boiling point (such as methanol, 2-propanol) is usually used as the dispersion solvent. However, such solvents are poor to wet and disperse single-wall carbon nanotubes. Comparing with such solvents, organic solvents such as tetrahydrofuran, dimethylformamide are slightly better in dispersion. But, the toxicity of tetrahydrofuran is high, and the boiling point of dimethylformamide is too high. The concentration of the dispersion liquid of carbon nanotube prepared by the method for dispersing single-wall carbon nanotubes by a single solvent is very low, so it is necessary to preprocess the single-wall carbon nanotubes before dispersion by conventional solvents, so as to improve its dispersing performance.